1. Field of the Invention
The present invention relates to an electronic warfare system and more particularly to a linear one-dimensional interferometer array and processing system for computing the angle of arrival of intercepted pulses, for example, radar pulses, for determining the location of the pulse emitter for targeting purposes; the system being more computationally and/or memory efficient than known systems.
2. Description of the Prior Art
Various electronic warfare systems are known in the art. Such electronic warfare systems are used in various applications including detecting the location of, for example, enemy radar devices. More particularly, such electronic warfare systems, also known as electronic support measures (ESM) systems, normally include a plurality of antennas configured in a linear one-dimensional array. These antennas are normally connected to one or more receivers, which, in turn, are connected to processing hardware for computing the angle of arrival of intercepted enemy radar pulses, for example. The processing hardware utilizes the phase differences of the intercepted pulses to determine their angle of arrival. The angle of arrival information allows the location of the enemy radar system to be ascertained. Such information may then be used for targeting the enemy radar system.
Examples of systems for computing the angle of arrival, for example, of radar pulses, are described in detail in U.S. Pat. Nos. 4,638,321; 4,977,635; 5,061,064; 5,497,161; 5,572,220; and 5,657,027, hereby incorporated by reference. Such systems are also disclosed in commonly owned U.S. Pat. Nos. 5,381,150; 5,451,956 and 5,572,213.
The phase difference between one antenna and another is not a monotonic function of the angle of arrival because the phase measurements are restricted to the 2xcfx80 interval between xe2x88x92xcfx80 and xcfx80. Roll-overs (multiples of +/xe2x88x922xcfx80) account for the difference between the xe2x80x9cideal phase differencexe2x80x9d (which does increase monotonically with increasing angle of arrival) and the measured phase difference. Thus, for example, an ideal phase value slightly larger than 3xcfx80 rolls over to a measured phase value slightly larger than xe2x88x92xcfx80. Some known systems estimate the angle of arrival by xe2x80x9ccomparingxe2x80x9d the vector of the measured phase differences between all the antennas with a set of hypothetical (i.e., noiseless) phase a difference vectors corresponding to different possible angles of arrival. To obtain an accurate estimate, many possible angles of arrival must be considered, and hence many xe2x80x9ccomparisonsxe2x80x9d must be made, requiring considerable computation or memory. Such known systems are thus too processor-intensive for use in a real-time pulse sorting/detection environment.
Circumstances in which the transmitting source characteristics change rapidly relative to the array (e.g., when the source is frequency-hopping or moving or when the array is moving in an unknown manner), or in which the source is rarely transmitting, cause additional problems for such known systems. In particular, known systems which require large amounts of data to estimate the angle of arrival would be at a disadvantage in such application, either because the data characteristics would significantly change during the time the data was being collected if the source characteristics were changing rapidly, or because a large amount of time would be required to collect the data if the source were rarely transmitting.
Briefly, the present invention relates to an electronic support measures (ESM) system which includes a plurality of antennas configured in a linear array forming a linear one-dimensional interferometer. The N+1 antennas in the array are spaced at locations 0, "xgr"d1, . . . , "xgr"dN in terms of units of a reference length "xgr"xe2x88x92where d1, . . . , dNxe2x88x921 are relatively prime numbers (i.e. numbers in which the largest common divisor is one) and di, dN is relatively prime for some i in the range 1xe2x89xa6i less than N. In order to enhance the computational efficiency and ability of the system, a Diophantine processing algorithm is utilized which is based on resolving the number of phase roll-overs. In particular, the method in accordance with the present invention for estimating the angle of arrival is based on determining the number of 2xcfx80 roll-overs which resulted in the measured phase difference between the reference antenna and some other antenna in the array, e.g., the antenna furthest away from the reference antenna. The present invention thus avoids comparisons to a table of hypothetical phase vectors as is done by known systems, directly solving for the angle of arrival after determining the number of phase roll-overs, and is thus suitable as an electronic warfare system in applications currently beyond the capability of known systems. The computational efficiency of the system is thus relatively improved and requires only N2 real multiplications, where N+1 is the number of antennas in the array. As such, the ESM system is relatively more efficient and requires reduced computation and/or memory relative to known systems. In addition, the ESM system in accordance with the present invention is particularly suitable to be used in various avionics applications that were heretofore known to be beyond the computational processing ability of known systems, for example, in applications in which the transmitting source characteristics change rapidly relative to the array or in which the source is rarely transmitting.